1. Field of the Invention
The present invention relates to fiber reinforced resin composites. In particular, the invention relates to the use of a partially impregnated preform comprising a layer of continuous fiber material having a partially impregnated resin on one or both faces of the fiber material which forms a monolithic composite upon curing.
2. Related Background Art
In recent years the use of high-strength-to-weight ratio fiber reinforced resin composites has continuously expanded, particularly in weight-sensitive products, such as aircraft and space vehicles. Fiber reinforced resin composites used in such products have usually been created by forming a layup, e.g., a stack of layers or plies, the layers or plies being formed of unidirectional or multidirectional (e.g., woven) fabrics made of glass or graphite fibers completely preimpregnated with a resin. Such plies preimpregnated with resin are commonly referred to as xe2x80x9cprepregxe2x80x9d plies or simply xe2x80x9cprepregxe2x80x9d. Normally the layup comprising the stack of layers or plies is positioned atop a forming tool, which, in its simplest form, may comprise a flat plate. After the layup is prepared, heat and pressure are applied. The heat cures the resin and the pressure compresses the layup preventing air and other gases, including volatile gases, from forming pores (bubbles) as the resin cures. Normally, an autoclave is used to apply the necessary heat and pressure.
While monolithic structures formed of fiber reinforced resin composites processed in the manner described above are satisfactory in some environments, they have certain disadvantages. For example, it has become desirable to provide cross-ply reinforcement in order to increase resistance to xe2x80x9cin-planexe2x80x9d compression load failure, particularly after limited input damage. In-plane loads are those loads lying in the plane of the plies. Cross-ply reinforcement (sometimes referred to as Z-direction reinforcement) is created by cross-ply stitching a layup. However, the implementation of cross-ply stitching has proven to be difficult to accomplish. The difficulty occurs because the prepregs are preimpregnated with resin, which is tacky. The resin makes it extremely difficult to cross-ply stitch the fiber layers together. The needle becomes contaminated or otherwise gummed up with the tacky resin making it extremely difficult to stitch the layers together. The needle also causes damage to the fibers in the layers.
Another disadvantage of using fiber plies preimpregnated with resin is the difficulty of removing gases trapped between the plies when a layup is formed and the gases are produced in the layup when the resin is being cured. While the pressure applied during curing forces most entrapped gases into solution, some bubbles still form, resulting in formation of weakening voids in the resultant monolithic structure.
A further disadvantage associated with the use of preimpregnated fiber plies is the need to store such plies at a low temperature and the losses associated with the failure to use such plies in a timely manner. More specifically, as will readily be appreciated by those familiar with resins used to date to create fiber reinforced resin composites, the rate of resin curing is accelerated when resin temperature is raised. Conversely, the rate of resin curing is retarded by low temperatures. As a result, conventionally, prior to use, preimpregnated fiber plies (which are usually in the form of relatively wide tape or fabric on rolls prior to being laid up) are stored in a refrigerated environment. Since the low storage temperature impedes resin curing, the usable life of preimpregnated fiber plies is increased. However, even at low temperatures resins may cure, albeit at a slower rate. As a result, at some point, even preimpregnated fiber plies stored at low temperature become unusable and must be disposed of. Even though the resin is the only portion of the preimpregnated fiber ply that becomes useless, the fiber as well as the resin must be disposed of because the resin has started to cure.
In the case of the production of a composite aircraft wing structure, damage-tolerance of the composite wing structure is enhanced by stitching together the fabric layers used to form the composite structure. In current prior art processes, stitching of the fabric layers must occur prior to resin preimpregnation of the fabric because the needle used to stitch conventional prepregs causes excessive damage to the resin impregnated fibers. In order to meet this problem, the desired number of fabric layers are stitched in the absence of the resin and then, during the final curing process, resin is forced through the entire thickness of the prestitched fabric layers using a resin film infusion (xe2x80x9cIRFxe2x80x9d) process. However, this approach leads to another problem. The resin must reach or infuse sufficiently to impregnate tall stiffeners in the wing structure to form a strong composite structure. Because it is very difficult to achieve full resin penetration to the vicinity of such stiffeners using these processes, it has been found that many anomalies exist in the resulting composite material.
A method and apparatus for creating monolithic structures formed of fiber reinforced resin composites, i.e., layers or plies bonded together by a cured resin is disclosed in U.S. Pat. No. 4,622,091. A plurality of dry plies are stacked to create a dry preform. The plies may or may not be stitched in the cross-ply direction. To form a composite a stack of dry preforms is created. After the stack is created, one or more layers of liquid or solid resin are added. The stack and the resin layer(s) are then cured under vacuum.
This method of creating monolithic structures suffers from the disadvantage that the plurality of dry plies must be stitched in the absence of the resin material. After stacking a plurality of stitched preforms to form a composite structure, the resin must then be hand placed between adjacent stitched dry preforms prior to infusion. This leads to increased manufacturing costs and production times.
The present invention is directed to avoiding the disadvantages of creating monolithic structures from preimpregnated fibrous layers with resins that require refrigeration or are not readily storable. More specifically, the invention is directed to a partially impregnated preform material that comprises a fabric layer partially impregnated with a resin which is stable against premature curing over long periods of time when stored at low temperatures. The partially impregnated preform is also stable at ambient temperatures when stored for shorter periods of time. The invention is also directed to fiber reinforced resin composites that are made from the instant partially impregnated preforms or a stack of preforms that can be easily cross-ply stitched in the presence of a resin film and that are formed in a manner that substantially reduces if not entirely eliminates weakening voids created by trapped gases by removing such gases prior to and during the infusion of the resin. Furthermore, the invention is directed to a process for preparing fiber reinforced resin composites that substantially reduces the amount of waste resulting from the premature curing of stored resins and the rejection of partially impregnated preforms due to poor quality. The invention is also directed to novel resin materials used in the inventive partially impregnated preforms.
The invention includes a partially impregnated preform comprising a fiber layer partially impregnated with a resin. The invention also provides for a partially impregnated preform comprising a plurality of fiber layers wherein one face of said plurality of fiber layers is partially impregnated with a resin. The invention further provides for a stack of partially impregnated preforms comprising a plurality of partially impregnated preforms wherein each partially impregnated preform comprises a fiber layer partially impregnated with a resin. The fiber layer for each of the partially impregnated preforms is formed of a plurality of parallel oriented tows, each tow formed of a plurality of unidirectional reinforcement fibers. The plurality of unidirectional reinforcement fibers may be selected from the group consisting of glass, quartz, organics such as KEVLAR(copyright) brand polyamide, carbon, graphite and the like. The resin is partially impregnated on one or both faces of the fiber layer. The resin is preferably a film, a powder or a liquid. The resin has the characteristic of being substantially tack free or non-tacky at ambient temperatures. The resin has preferably a minimum viscosity from about 0.5 poise to about 1000 poise at temperatures from about 50xc2x0 C. to about 400xc2x0 C. The partially impregnated preform or plurality of preforms may be cross-ply reinforced by cross-ply stitching the partially impregnated preform(s).
A method of forming a fiber reinforced resin composite using the instant partially impregnated preform(s) comprising the steps of:
(a) enclosing a partially impregnated preform in a resin content control envelope, said partially impregnated preform comprising a fiber layer partially impregnated with a resin;
(b) enclosing said partially impregnated preform in said resin content control envelope in a vacuum envelope;
(c) evacuating said vacuum envelope and said resin content control envelope to withdraw air and other gases from said partially impregnated preform; and
(d) heating said partially impregnated preform simultaneously with the evacuation of said vacuum envelope and said resin content control envelope to cause said resin to melt, to fully infuse into said fiber layer and, then, to cure as air and other gases are withdrawn from said fiber layer resulting in the formation of said fiber reinforced resin composite.
The method may include the step of: positioning a second partially impregnated preform atop said partially impregnated preform of step (a).
The partially impregnated preform or plurality of partially impregnated preforms used in this method may be cross-ply reinforced by cross-ply stitching the preform(s) together. In addition, the method may include a step of placing a plurality of doubler layers (i.e, either stitched or unstitched fabric layers) atop the partially impregnated preform(s) or including the step of positioning a core between said partially impregnated preform(s). The core may be a honeycomb core. The partially impregnated preform(s) and the core may be cross-ply reinforced by cross-ply stitching the core and the partially impregnated preform(s) together. The fiber reinforced resin composite prepared according to the disclosed method may be used to form a material for an aircraft or space vehicle.
The invention also includes a resin composition for partially impregnating a fiber layer of a preform comprising: (a) from about 90 to about 99 weight percent of at least one epoxy resin; and (b) from about 1 to about 10 weight percent of a curing agent, and the composition is capable of being stitched after such partial impregnation and exhibiting reduced viscosity upon heating to fully infuse said fiber layer upon curing. The epoxy resins may comprise about 10.7 weight percent having the following structure, 
about 61.0 weight percent having the following structure, 
xe2x80x83and about 24 weight percent having the following structure, 
xe2x80x83and said curing agent is about 4.3 weight percent of a curing agent such as a cyanoguanidine. The cyanoguanidine may be DICYANEX 1400B.
As will be readily appreciated from the foregoing description, the invention provides a new and improved method for creating monolithic structures formed of fiber reinforced resin composites. The method overcomes the disadvantages of previous prepreg methods used to form monolithic structures. Specifically, because the preform is partially impregnated with a resin substantially tack-free, cross-ply stitching of a stack of partially impregnated preforms is facilitated. Furthermore, stitching of the stack of partially impregnated preforms occurs without damage to the fiber material unlike traditional prepreg methods where damage occurs to the fiber material during stitching of the prepreg. A significant advantage of the use of a stack of partially impregnated preforms to form a composite structure is that each partially impregnated preform may have resin intimately associated with each layer (i.e. partially impregnated resin). Thus, complete resin infusion or wetting of the final composite material is significantly increased since the distance the resin must travel to completely wet the composite material is significantly reduced.
In the prior art layups it was common to stitch a stack of preforms in the absence of resin and then to place the stack of stitched preforms on the resin material. During curing the resin was required to travel from the bottom of the stack to the top of the stack to completely wet out the final composite material. Also, in the traditional approaches used to form composite structures, the viscosity of the resin was reduced by heating and by applying pressure. Only when viscosity was reduced sufficiently.was it possible to force the resin into a thick stack of layers of fiber materials. Significant problems could be encountered due to the relatively short time in which cross-linking of the resin occurs. Upon cross-linking the viscosity dramatically increased and, thus, complete wet-out of the composite structure was not obtained. As a result of incomplete wet-out of a composite structure the strength and toughness of the material is compromised.
Another advantage of the present method used to form the composite structure is that the resin flows into the dry fiber material during curing while a gas path is still provided through the fibers and out of the resin content control envelope. Accordingly, gases (including volatile gases) are not trapped in the fiber reinforced resin composite as it is being formed. In addition, because the resin in the partially impregnated preforms has a relatively long shelf-life, the partially impregnated preforms can be conveniently stored at low temperatures (i.e., refrigerated) for considerable periods of time without premature curing, until it is required for use.
Thus, the present invention provides a method to partially impregnate a fabric layer to form a partially impregnated preform which permits stitching of a plurality of partially impregnated preforms without excessive fiber damage and needle contamination. The partially impregnated resin on the fabric layers is able to flow throughout the laminate during final curing. Another advantage of the present method is that the resin in the partially preimpregnated preform(s) does not deeply saturate the fabric layers of the preform unlike existing prior art prepregs which make reinforcement by stitching very difficult.